Place the ricotta, celery, parsley and cayenne pepper in a bowl and mix well. Refrigerate until needed. Preheat the oven broiler to high.
Leave any small mushrooms whole and halve larger ones. Place them in a large, heavy, non-stick skillet and add the garlic, thyme, sour cream, and 1 tsp (5 ml) water. Cover and cook gently until the mushrooms are just tender and have given up their juices, about 3 to 4 minutes. Add the lemon juice and season lightly with salt and pepper.
While the mushrooms are cooking, toast the bread slices on both sides under the broiler. While still warm, spread one side of each piece of toast with some of the ricotta mixture, then cut it in half.
Arrange the toasts on individual serving plates. Spoon the hot mushroom mixture over the toasts and serve immediately.
Nutrients per serving: 148 calories, calories from fat 36, 4 g fat (1 g saturated fat), 6 mg cholesterol, 24 g carbohydrate (4 g fibre), 3 g sugar, 8 g protein